David Miliband has spoken of his "frustration" at Labour being in opposition while a "pretty hard-right" Government is at the helm.
The elder brother of Labour leader Ed Miliband said the party had to be "humble" about its "mistakes", but proud of what it had achieved in power.
His comments - a rare intervention in domestic politics since he lost out on the leadership to his younger brother - came in an interview with Indian newspaper The Hindu.
"I think for everyone in the Labour Party, it feels very frustrating at the moment. Because opposition is a permanent lesson in frustration; you can talk but you can't do anything," he told the newspaper.
"Britain now has quite a radical Government. Some people thought that a coalition Government would be centrist. In fact, this is a pretty hard-right Government in economic terms."
The former foreign secretary said the impact of the Government's policies on unemployment and poverty in his South Shields constituency was "pretty painful", adding: "So the whole Labour Party is frustrated by opposition.
"Equally you can't be in government forever. We had thirteen years."
Miliband did not comment directly on his brother's leadership, which has been under the spotlight over the past week.
But he said Labour, in trying to get back into power, had to challenge the Conservatives in "an ideological and intellectual and political way".
"There's a debate in the Labour Party about how we should understand our record in government, what we should be proud of and what we should apologise for," he said.
Miliband added that it was "very important to be proud of your achievements and humble about your mistakes", while always understanding that politics is about the future.
"So we have a responsibility to understand the fundamental ways in which the world is changing and Britain's place in the world is changing. And make sure we are able to challenge the Conservatives because in the end, it's them who we have to challenge," he added.
"It's a coalition Government but it's the Conservatives who are the real enemy. We have to challenge them in an ideological and intellectual and political way."Suggest a correction